The phenomenon of splitting of white light into its constituent colours is called dispersion of light. The band of seven colours obtained from the splitting of white light is called the spectrum.
All the constituent colours of a white light have same velocity in vacuum, but when it passes through a transparent medium like glass prism, their velocity changes. At the first face of the prism, different colours are deviated by different angles. Violet having the minimum speed gets deviated by maximum angle and red having the maximum speed gets deviated by minimum angle. Thus, the colours ' P ' and ' Q ' are violet and red respectively.